1. Field of the Invention
This invention relates generally to oil and gas wells, particularly subsea wells. In particular, the invention relates to a tieback casing string provided in offshore petroleum production installations for providing a protective barrier and a fluid conduit between a subsea wellhead and a surface wellhead located on an offshore drilling or completion platform. Still more particularly, this invention relates to a slip type well casing hanger, for suspending a tieback casing string from a subsea wellhead to a surface wellhead located on an offshore drilling or completion platform.
2. Description of the Prior Art
Prior arrangements to tieback a subsea wellhead to the surface wellhead of a subsea well have been complicated, requiring a great amount of equipment and high installation cost. Furthermore, conventional slip type hangers potentially cause fatigue failure of the casing string suspended with a conventional slip type hanger.
The sharp, hardened teeth of conventional casing hanger slips produce a series of circumferential grooves around the outside diameter of the casing. The bottom of these grooves has very sharp points which act as stress risers. On a floating offshore platform, the conventional casing string is continuously moving due to ocean waves and currents. The sharp teeth of the casing hanger slips become a pivot or fulcrum point for side bending loads due to horizontal movement. Such side bending load is supported by the sharp teeth of the slip, and these teeth continuously produce deeper and deeper circumferential grooves, thus creating a potential for fatigue failure.
Various wellhead supply companies have supplied a type of adjustable casing hanger to circumvent the potential of fatigue failure with conventional slip type hangers. These companies include ABB Vetco Gray, Cameron, FMC and Drilquip. The prior adjustable casing hangers are expensive requiring complex and expensive running tools.